All nuclei with odd number of protons or neutrons have an impulse moment or spin different from zero. The nuclei also have a positive electric charge which, together with the spin of the nucleus, produces for the nucleus a magnetic moment whose direction coincides with the spin axis of the nucleus. A field generated by the magnetic moment of a nucleus can be approximated by the field of a magnetic dipole. If a sample containing a plurality of nuclei is placed in a static magnetic field, the magnetic moments of the nuclei tend to align parallel with an external magnetic field and the sample will be provided with a net magnetization parallel to the external magnetic field. The magnitude of the net magnetization is proportional to the number of nuclei in a sample and to the strength of an external magnetic field. The orientation of nuclei is also disturbed by the thermal motion of nuclei, so the magnitude of the magnetization will also be affected by the temperature of a sample: temperature increases, magnetization decreases. In terms of quantum mechanics, the events can be described so that an external magnetic field generates a number of energy levels depending on the spin quantum number (I) of a nucleus, at which levels a nucleus can set with a certain probability. The nucleus of a hydrogen atom or proton has a spin quantum number I=1/2, so a proton can settle at two energy levels either in a manner that the direction of its magnetic moment is the same as that of the external magnetic field or reverse to this direction. Of these two, the former is more probable and the occupation proportions of energy levels conform with the so-called Boltzmann distribution. In order to move from one energy level to another, a nucleus either receives or delivers an energy quantum as electromagnetic radiation of a certain frequency. The radiation frequency is determined by the difference between energy levels, which is directly proportional to the strength of the external magnetic field. This frequency associated with exchange of energy is called the Larmor frequency. This exchange of energy between nucleus and environment is called the nuclear magnetic resonance phenomenon. The principles of neclear magnetic resonance have been dealt with in e.g. the following references: Abragam A: The principles of nuclear magnetism, London Oxford University Press, 1961 and Slichter CP: Principles of magnetic resonance, Berlin, Springer Verlag, 1981.
The nuclear magnetic resonance phenomenon has been studied by so-called continuous radiation (CW, Continuous Wave) and pulse methods. Pulse methods have been found more effective than CW-methods and are thus employed in NMR-spectroscopy and so-called nuclear spin imaging (magnetic resonance imaging, MRI). In pulse methods, a sample is subjected to an electromagnetic pulse of Larmor frequency, whose duration is determined in a manner that the nuclear magnetization of a sample tilts through a desired angle (.nu.) relative to the direction of the external magnetic field. The amplitude and duration of an electromagnetic pulse are generally selected in a manner that (.nu.) is a multiple of 90 degrees. Generally used terms are 90-degree and 180-degree pulses etc. The net magnetization M.sub.n deflected from the direction of the external magnetic field B.sub.o after the act of excitation precesses at the Larmor frequency w.sub.o around the direction of B.sub.o. This can be established by placing a coil outside a sample in a manner that its magnetic axis is orthogonal to the direction of B.sub.o. The precessing net magnetization induces in the coil a so-called FID-signal (Free Induction Decay), which has Larmor frequency and whose amplitude is proportional to the strength of the nuclear magnetization of the sample and to the strength of the external magnetic field.
During the excitation, a nuclear system receives external energy from an excitatary radio-frequency field, and after the excitation, delivers it to its environment. The emission of energy may occur as coherent radiation that can be detected by means of an external coil or energy may be transferred into the structure of a sample as thermal motion. In connection with the delivery of energy, the net magnetization of a sample returns to its equilibrium value. The nature of this process is exponential and characterized by a relaxation time T.sub.1. This relaxation time is dependent on the composition of a substance to be examined: e.g. with liquid substances T.sub.1 is relatively short (milliseconds to seconds), while with solid substances T.sub.1 is long (minutes to weeks). The coherence of radiation emitted by a sample declines after excitation at a rate determined e.g. by the properties of a substance to be examined and by the homogeneity of an external magnetic field. This results in exponential decay of a signal at a rate characterized by a relaxation time T.sub.2 * (T.sub.2 star). EQU 1/T.sub.2 *=1/T.sub.2 +.gamma..times..DELTA.B.sub.o /(2.times..pi.), (1)
wherein T.sub.2 is the spin-spin relaxation time of a sample
.gamma. is a gyromagnetic ratio PA1 .DELTA.B.sub.o is the inhomogeneity of a polarizing magnetic field over a sample.
The relaxation time T.sub.1 .rho. is analogous to T.sub.1 longitudinal relaxation time and is the longitudinal relaxation time in a rotating frame of reference. This frame of reference rotates around the z' axis at the Larmor angular speed. The z' axis coincides with the z axis in the conventional reference frame xyz. The external magnetic field B.sub.o is directed along the z' axis. The effective static field due to the external magnetic field in this frame of reference is zero. The magnetic component of the excitatory RF field appears as a static field along the x axis. In the T.sub.1 .rho. technique, the nuclear magnetization M.sub.o is tilted 90.degree. from the direction of the external magnetic field by an RF signal. Shortly thereafter, a RF field which has a magnetic component along the tilted magnetization is applied (locking field). The magnetization of the spin system is now locked by the RF field. The nuclear magnetization relaxes toward the thermal equilibrium value corresponding to the strength of the magnetic component of the locking field. Because the magnetic component of the locking field is much smaller than the external field, this equilibrium value is approximately zero. T.sub.1 .rho. is sensitive to low frequency processes in a sample and T.sub.1 is sensitive to processes that are higher in frequency. Because of the greater efficiencies at lower frequencies, T.sub.1 .rho..ltoreq.T.sub.1. All above relaxation times are dependent on the immediate environment of nuclei and its activity. As pointed out above, the physical state of a sample has an effect on relaxation times but also the strength of an external magnetic field and temperature of a sample change relaxation times. The usefulness of the nucleus of a hydrogen atom or the proton in medical diagnostics is based on the abundance of hydrogen in soft tissues, in which it is primarily bound to water molecules. By virtue of its polarity, in turn, a water molecule links itself in various ways to different protein chains and this linkage is altered for a plurality of reasons, e.g. due to a pathological process directed at a tissue. Relaxation times and their alterations have been dealt with in e.g. the following references: R Damadian U.S. Pat. No. 3 789 832 and Nuclear Magnetic Resonance of Intact Biological Systems, Phil Trans R Soc Lond, 289, June 1980.
Interest in utilizing the nuclear magnetic resonance phenomenon in medicine rose in the early 1970s. That was when R. Damadian published his research results, revealing that the relaxation time T.sub.1 of a malignant tumour tissue is even twice as long as that of a normal tissue. The publication R. Damadian U.S. Pat. No. 3 789 832 discloses a method for identification of a malignant tumour tissue by comparing the measured relaxation time of a tissue with tabulated relaxation time values and then diagnosing possible malignancy of a specimen. However, later studies have indicated that the changing of relaxation times is not specific of any particular pathological condition. It can be generally concluded, however, that relaxation times change readily due to various ailments and can thus be applied in medical diagnostics. The publication U.S. Pat. No. 3 789 832 also discloses a kind of scanning apparatus for the examination of a human body by means of NMR. However, this prior art solution cannot be regarded as any spin imaging apparatus. The basic idea of nuclear spin imaging was published by PC Lauterbur in 1973 in the reference PC Lauterbur: Nature, 242, 190, 1973. In this publication, he also brought up the idea of mapping a relaxation time T.sub.1. Several pulse sequences have been developed for measuring relaxation times, including a so-called Saturation Recovery and Inversion Recovery sequences for measuring T.sub.1 and Spin-Echo-sequence for measuring T.sub.2. These sequences have been dealt with e.g. in the reference: Farrar TC, Becker ED: Pulse and Fourier Transform NMR-Introduction to Theory and Methods, Academic Press, New York, 1971.
The nuclear spin imaging methods can be roughly classified in three categories: 1. Point imaging, 2. Line imaging and 3. Volume imaging methods. In point imaging techniques, an object area to be examined is mapped by moving the object or a point-like area obtained by various technical means for generating an NMR signal with respect to each other. The main disadvantage of single point techniques is that they are slow and, therefore, they are not applied in medical imaging. With special arrangements, however, the point imaging methods can be used to obtain more tissue information than with e.g. whole volume imaging methods. The single point imaging techniques have been described e.g. in references: Tanaka et al: IEEE, vol. 66, No. 11, 1582-1583, 1978, Damadian: Offenlegungschrift No. 29 46 847, Moore et al: U.S. Pat. No. 4 015 196, Abe: U.S. Pat. No. 3 932 805, Garroway et al: U.S. Pat. No. 4 021 726, Crooks et al: U.S. Pat. No. 4 318 043, Young: UK Pat. Appln GB No. 2 122 753 A. By combining the slow single point imaging technique and rapid ultrasonic imaging technique, as set forth in the reference Sepponen: FI Pat. No. 64282, the single point imaging techniques can be utilized in medical diagnostics.
Line imaging techniques have been described e.g. in the following references: Moore et al: U.S. Pat. No. 4 015 196, Sepponen: FI Pat. No. 58868, Garroway et al: U.S. Pat. No. 4 021 196, Crooks et al: U.S. Pat. No. 4 318 043, Hutchinson et al: U.S. Pat. No. 4 290 019.
The line imaging methods are also too slow for medical imaging and, thus, their application is restricted to certain special cases.
Imaging of a three-dimensional object is most preferably effected by applying whole volume imaging techniques. By means of so-called selective excitation, it is possible to define the object area to be examined from an object and to effect a more accurate mapping of the distribution of NMR parameters. Selective excitation can be effected by exciting over the object a magnetic field gradient orthogonal to the plane of an object area to be excited and by modulating an exciting radio-frequency pulse in a manner that its frequency band width and the gradient field strength correspond to the width of a desired object area. Another method of defining an object area is to utilize a oscillating magnetic field gradient, as set forth in the reference: Moore et al: U.S. Pat. No. 4 015 196. Previously known is also to utilize a gradient in an exciting radio-frequency pulse in a manner that on successive times of excitations the gradient direction is changed, a stable NMR signal being only generated in the plane where the pulse amplitude is constant.
A considerably more inaccurate method is to utilize the geometrical properties of transmitter and receiver coils for defining an object area and, thus, this method has only been applied when it is desired to effect NMR spectroscopic studies of the object. The application of this method has been described in references: Ackerman et al: Nature 283, 167, 1980, Haase et al: J. Magn. Reson. 56, 401-412, 1984, Bottomley et al: Radiology, 150, 441-446, 1984.
Whole volume imaging methods have been described e.g. in references: Lauterbur: Nature, 242, 190-191, 1973, Ernst: U.S. Pat. No. 4 070 611, Hutchinson et al: International Patent application WO No. 81/02788, Sepponen: FI application No. 824343. For speeding up the imaging, it is possible to apply methods disclosed in references: Edelstein et al: GB application No. 2 079 463, Mansfield: U.S. Pat. No. 4 165 479, Hinshaw: Physics Letters 48A, No. 2, June 3, 87-88, 1974, Likes: U.S. Pat. No. 4 307 343. Particularly noteworthy nuclear spin imaging methods are so-called Fourier imaging methods, one version of which has been set forth in the reference Ernst: U.S. Pat. No. 4 070 611. A drawback in this cited method is the collection of an FID-signal generated after the excitation pulse. Encoded in the phase of a collected FID-signal is the position information of one or two perpendicular directions by means of gradient pulses of constant amplitudes but varying durations. A drawback in this method is e.g. that the moment of pick-up changes on various signal pick-up times, resulting in the method's sensitivity to the inhomogeneities of a polarizing magnetic field B.sub.o and, thus, T.sub.2 of a sample or specimen also affects the signal to be picked up. The reference Hutchison et al: WO No. 81/02788 discloses a variation of Fourier imaging technique for generating a sort of spin echo by changing the magnetic field gradient direction. This spin echo is stored and its phase is encoded with a gradient pulse orthogonal to the direction of a read-out gradient, the amplitude of said gradient pulse being varied on various repetition cycles. A more preferred way of generating a spin echo is to compensate the effect of the basic field inhomogeneities on a final image. Applications of this method have been described in references: Edelstein et al: EP 91008, Bottomley et al: EP 98426, Hutchison et al: Proceedings of 18th Ampere Congress, Nottingham, 1974, 283 . . . 284 and Sepponen: FI Appln. No. 824343 corresponding to U.S. Pat. Application No. 561,339, filed Dec. 13, 1983, now U.S. Pat. No. 4,654,594 of Mar. 31, 1987. The reference Brunner P: Journal of Mag. Res. 33, 83-106, (1979) discloses how to speed up the examination of a three-dimensional object with nuclear spin imaging techniques by directing the excitation and detection phases in temporal succession at various parts of an object. This serves to avoid a long duration of the examination caused by the nuclear system recovery time.
At present, nuclear spin imaging is primarily used for mapping the hydrogen distribution of an object and the distributions of the relaxation times T.sub.1 and T.sub.2 of a nucleus. On the other hand, methods applicable to the mapping of relaxation time T.sub.1 .rho. have not been published. The method of the invention and the apparatus for carrying out the method are used for mapping the distribution of relaxation time T.sub.1 .rho. of an object, such as a human body, trunk of a tree or a nutritive substance.